The periods of physiological activity, such as the respiratory and cardiac periods, have an important effect on synchronizing the image frames in the image sequences of the physiological activity. The phase of an individual frame in an image sequence of the periodic physiological activity provides critical intermediate information for medical imaging and diagnosis. For example, when the doctor interprets pulmonary CT image sequences, he/she needs to mark up the exhale phase and inhale phase. For another example, when reconstructing a 3D image for a physiological structure using 2D images acquired from various scanning angles, it is necessary for the 2D images to have the same phase in the periodic physiological activity. For another example, in the field of cardiovascular diagnosis and treatment, accurately determining the phase of individual frame in the cardiac period facilitates deriving other important physiological parameters, such as heart rate, cardiac period, etc.
Conventionally, separate periodic-monitoring devices are used, such as electrocardiograph, respiratory management system, etc., for recording and monitoring the related clinical information. For example, an ECG gating device is typically used to generate a synchronizing ECG gating signal, in order to correlate the image frames and their cardiac phases. However, these monitoring devices are not part of the standard configuration for most common imaging systems, and the additional devices occupy extra space at the imaging scene.
Extant methods for evaluating the cardiac periodic phase on the basis of image sequence of vessel generally include the following. One finds the time window with the slowest phase change by calculating the difference between different frames (e.g., adjacent frames) to perform the analysis using several image frames within the time window. This method does not evaluate the phase of each frame and cannot distinguish the diastole from the systole. In addition, this method is sensitive to background noise in the image. Another method is to determine the phase on the basis of comparison to a reference image. This method requires manual selection of the reference image at the desired phase or assumes a high similarity between a reference image in a database and the present image sequence. This method suffers from the same problems when used for image sequences for other periodic physiological activity other than the cardiac activity, such as pulmonary activity in the respiration period.